An alternative separation technology called supercritical fluid chromatography (SFC) has advanced over the past decade. SFC uses highly compressible mobile phases, which typically employ carbon dioxide (CO2) as a principle component. In addition to CO2, the mobile phase frequently contains an organic solvent modifier, which adjusts the polarity of the mobile phase for optimum chromatographic performance. Since different components of a sample may require different levels of organic modifier to elute rapidly, a common technique is to continuously vary the mobile phase composition by linearly increasing the organic modifier content. This technique is called gradient elution.
SFC has been proven to have superior speed and resolving power compared to traditional HPLC for analytical applications. This results from the dramatically improved diffusion rates of solutes in SFC mobile phases compared to HPLC mobile phases. Separations have been accomplished as much as an order of magnitude faster using SFC instruments compared to HPLC instruments using the same chromatographic column. A key factor to optimizing SFC separations is the ability to independently control flow, density and composition of the mobile phase over the course of the separation. SFC instruments used with gradient elution also reequillibrate much more rapidly than corresponding HPLC systems. As a result, they are ready for processing the next sample after a shorter period of time. A common gradient range for gradient SFC methods might occur in the range of 2% to 60% composition of the organic modifier.
It is worth noting that SFC instruments, while designed to operate in regions of temperature and pressure above the critical point of CO2, are typically not restricted from operation well below the critical point. In this lower region, especially when organic modifiers are used, chromatographic behavior remains superior to traditional HPLC and often cannot be distinguished from true supercritical operation.
A second analytical purification technique similar to SFC is supercritical fluid extraction (SFE). Generally, in this technique, the goal is to separate one or more components of interest from a solid matrix. SFE is a bulk separation technique, which does not necessarily attempt to separate individually the components, extracted from the solid matrix. Typically, a secondary chromatographic step is required to determine individual components. Nevertheless, SFE shares the common goal with prep SFC of collecting and recovering dissolved components of interest from supercritical flow stream. As a result, a collection device suitable for preparative SFC should also be suitable for SFE techniques.
Packed column SFC uses multiple, high pressure, reciprocating pumps, operated as flow sources, and independent control of system pressure through the use of electronic back pressure regulators. Such a configuration allows accurate reproducible composition programming, while retaining flow, pressure, and temperature control. Reciprocating pumps are generally used in supercritical fluid chromatography systems that use a packed chromatography column for elution of sample solutes. Reciprocating pumps can deliver an unlimited volume of mobile phase with continuous flow, typically pumping two separate flow streams of a compressible supercritical fluid and incompressible modifier fluid that are combined downstream of the pumping stages to form the mobile phase. Reciprocating pumps for SFC can be modified to have gradient elution operational capabilities.
A great deal of subtlety is required to pump fluids in SFC. Not any reciprocating pump can be used with a pump head chiller to make an SFC pump. While most HPLC pumps can be set to compensate for the compressibility, compensation is too small to deal with the fluids most often used in SFC. To attempt to minimize the compressibility range required, the pump is usually chilled to insure the fluid is a liquid, far from its critical temperature. Chilled fluids are dense but are still much more compressible than the normal liquids used in HPLC. To control flow accurately, the pump must have a larger than expected compressibility compensation range. Further, since the compressibility changes with pressure and temperature, the pump must be capable of dynamically changing compressibility compensation. Inadequate compensation results in errors in both the flow rate and the composition of modified fluids.
Without correct compressibility compensation, the pump either under- or over-compresses the fluid causing characteristic ripples in flow and pressure. Either under- or over-compression results in periodic variation in both pressure and flow with the characteristic frequency of the pump (ml/min divided by pump stroke volume in ml). The result is noisy baselines and irreproducibility. To compensate for this, the more expensive and better liquid chromatography pumps have compressibility adjustments to account for differences in fluid characteristics.
SFC systems in the prior art have used modified HPLC high-pressure pumps operated as a flow source. One pump delivered compressible fluids, while the other was usually used to pump modifiers. A mechanical back pressure regulator controlled downstream pressure. The pumps used a single compressibility compensation, regardless of the fluids used. The compressible fluid and the pump head were cooled near freezing. The delivery of carbon dioxide varied with pressure and flow rate. The second pump delivered accurate flows of modifier regardless of pressure and flow. At different pressures and flows, the combined pumps delivered different compositions although the instrument setpoints remained constant. Pumping compressible fluids, such as CO2, at high pressures in SFC systems while accurately controlling the flow, is much more difficult than that for a liquid chromatography system. SFC systems use two pumps to deliver fluids to the mobile phase flow stream, and each pump usually adds pressure and flow ripples and variances that cause baseline noise. The two pumps also operate at different frequencies, different flow rates, and require separate compressibility compensations, further adding to the complexity of flow operations.
Methods in the prior art calculate ideal compressibility based on measured temperature and pressure using a sophisticated equation of state. The method then uses dithering around the setpoint to see if a superior empirical value can be found. This approach is described in U.S. Pat. No. 5,108,264, Method and Apparatus for Real Time Compensation of Fluid Compressibility in High Pressure Reciprocating Pumps, and U.S. Pat. No. 4,883,409, Pumping Apparatus for Delivering Liquid at High Pressure. Other prior art methods move the pump head until the pressure in the refilling cylinder is nearly the same as the pressure in the delivering pump head. One method in U.S. Pat. No. 5,108,264 Method and Apparatus for Real Time Compensation of Fluid Compressibility in High Pressure Reciprocating Pumps, adjusts the pumping speed of a reciprocating pump by delivering the pumping fluid at high pressure and desired flow rate to overcome flow fluctuations. These are completely empirical forms of compressibility compensation. The prior art methods require control of the fluid temperature and are somewhat limited since they does not completely compensate for the compressibility. The compensation stops several hundred psi from the column inlet pressure.
In SFC, it is common to use very long columns with large pressure drops to generate very high efficiency compared to HPLC. The use of long columns resulted from a change in control philosophy. Earlier in SFC technology, the pump was used as the pressure controller. the column outlet pressure was not controlled. Long columns produced large pressure drops, and at modest inlet pressures, the outlet pressure could drop to the point where several sub-critical phases could exist. The co-existence of several phases destroys chromatographic separations and efficiency. Controlling the column outlet pressure, the pump becomes a flow source, not a pressure source. Consequently, the point in the system with the worst solvent strength becomes the control point. All other positions in the system have greater solvent strength. By controlling this point, problems associated with phase separations or solubility problems at uncontrolled outlet pressures are eliminated.
The compressibility of the pumping fluid directly effects volumetric flow rate and mass flow rate. These effects are much more noticeable when using compressible fluids such as carbon dioxide in SFC rather than fluids in liquid chromatography. The assumption of a constant compressibility leads to optimal minimization of fluid fluctuation at only one point of the pressure/temperature characteristic, but at other pressures and temperatures, flow fluctuations occur in the system.
The flow rate should be kept as constant as possible through the separation column. If the flow rate fluctuates, variations in the retention time of the injected sample would occur such that the areas of the chromatographic peaks produced by a detector connected to the outlet of the column would vary. Since the peak areas are representative for the concentration of the chromatographically separated sample substance, fluctuations in the flow rate would impair the accuracy and the reproducibility of quantitative measurements. At high pressures, compressibility of solvents is very noticeable and failure to account for compressibility causes technical errors in analyses and separation in SFC.
The type of pump control philosophy in an SFC system affects resolution in pressure programming. A pressure control pump with a fixed restrictor results in broadened peaks and higher background noise through a packed column. Efficiency degrades as pressure increases. A flow control pump with a back-pressure regulator has better resolution results through a packed column and steady background. Efficiency remains constant with increasing pressure. With independent flow control, the chromatographic linear velocity is dictated by the pump, and remains near optimum, throughout a run. The elution strength is controlled separately, using a back-pressure regulator. With pressure controlled pumps, a fixed restrictor passively limits flow. The linear velocity increases excessively during a run, thereby degrading the chromatography.
Therefore, a need exists for a system that uses a pump as a pressure source in SFC without degrading the chromatography results.